1. Field of Invention
The present invention relates to a method for recording to and reproducing information from an optical recording medium comprising a recording layer that records and reproduces information by irradiation with a laser beam or the like, and to an optical recording medium and a recording and reproducing apparatus for the same.
2. Description of the Related Art
Optical recording media have garnered attention as high-volume and high-density storage devices, and come in a rewritable type that allows information to be rewritten, and a write-once type that allows information to be recorded only one time.
One such rewritable type of optical recording medium comprises a substrate over which is provided a recording layer consisting of a thin film that undergoes a phase change between amorphous and crystalline states, and information is recorded or reproduced with this medium by means of thermal energy produced by irradiation with a laser beam. Alloy films whose main component is germanium, antimony, tellurium, indium, or the like, such as a GeSbTe alloy, are known as phase-change materials that can be used for such recording layers. The recording of information is accomplished by forming marks by rendering parts of the recording layer amorphous, while the erasure of information is accomplished by crystallizing the amorphous marks. The recording layer can be amorphized by heating it to over its melting point and then cooling it at a certain rate or higher. Crystallization can be effected by heating the recording layer to over its crystallization temperature but under its melting point. Information is reproduced by utilizing the difference in reflectivity between these amorphous marks and the crystalline regions.
One write-once type of optical recording medium is a write-once type of recording medium in which an organic colorant film formed by spin coating is used as the recording layer. As the processing capability of various kinds of information device has risen in recent years, so too has the amount of information being handled, and less expensive recording media have come to be required. A write-once recording medium is an example of an inexpensive recording medium.
Both types of mediums generally comprise a substrate provided with spiral or concentric circular grooves for tracking a laser beam during recording and reproduction. The regions between the grooves are called lands, and usually the grooves constitute information tracks and the lands serve as guard bands for separating adjacent information tracks. With a rewritable Blu-ray disc, for example, the grooves are convex on the side where the laser beam is incident, and information is recorded by irradiating these convex portions with a laser beam. This configuration is referred to as on-groove recording. This is distinguished from a configuration in which information is recorded in grooves that are concave on the side where the laser beam is incident, and this is referred to as in-groove recording. FIG. 7 is a schematic diagram of a conventional disk cross section, illustrating the differences between on-groove recording and in-groove recording. FIG. 7a shows on-groove recording, and FIG. 7b shows in-groove recording. Grooves 702 are formed in a substrate 701. An information recording layer 703 is formed over the substrate 701, and a cover layer 704 with optical transparency is formed over this. A laser beam for recording and reproducing information is incident from the transparent layer side, and is focused on the groove 702 portion of the information recording layer 703. The grooves 702 are convex on the laser incident side in FIG. 7a, and are concave in FIG. 7b. 